To study the basic mechanical property of the new cast-in-place reinforced-concrete honeycombed-core girderless floor, experiments were performed for large-scale loading on reinforced-concrete honeycombed-core girderless floor, simply supported on four edges. Further, finite-element analysis for the honeycombed-core floor was conducted using ANSYS software. The overall analysis indicated that deformation of the floor at the elastic stage was relatively small. On the other hand, the overall rigidity and cracking resistance of the floor were high, which indicated that the mechanical behavior of the entire slab could be improved by increasing the width of the flat beam. The proposed floor design can be extensively applied to the construction work because of its good mechanical behavior.